In an optical communication field and an optical measurement field, an optical modulator is used. As the optical modulator, an optical modulator in which an optical waveguide and a control electrode for controlling light waves propagating through the optical waveguide are formed on a substrate having an electro-optic effect, such as lithium niobate (LN), is used. As shown in Patent Literature No. 1, an optical modulator having a configuration in which a modulation electrode which inputs a RF modulation signal and a DC electrode which applies a DC bias voltage are separately formed as a controlled electrode, is known.
Further, as a substrate configuring an optical modulator, there are an X-cut type substrate and a Z-cut type substrate, and in the X-cut type substrate, an electric field in a direction parallel to the surface of the substrate is applied to an optical waveguide, and in the Z-cut type substrate, an electric field in a direction perpendicular to the surface of the substrate is applied to an optical waveguide. In particular, in the Z-cut type substrate, it is necessary to form a control electrode directly above the optical waveguide, and in order to prevent light waves propagating through the optical waveguide from being absorbed by the control electrode, a buffer layer is provided between the optical waveguide and the control electrode.
In contrast, in the X-cut type substrate, control electrodes are disposed so as to put both side of the optical waveguide therebetween, and therefore, it is not necessary to provide a buffer layer. However, resulting from the wiring pattern of a feeder electrode of the control electrode, the feeder electrode inevitably crosses the optical waveguide, and therefore, some of light waves propagating through the optical waveguide are absorbed by the feeder electrode.
FIG. 1 is a diagram showing an example of a DC electrode formed on the X-cut type substrate. FIG. 1 shows an optical modulator in which two Mach-Zehnder type waveguides are disposed side by side. For example, a part of a nested optical waveguide is shown in which sub-Mach-Zehnder type optical waveguides (branched waveguides of the sub-Mach-Zehnder type optical waveguides are A1 and A2, and B1 and B2) are incorporated into two branched waveguides (A3 and B3) of a main Mach-Zehnder type optical waveguide in a nested manner.
At the Mach-Zehnder type optical waveguides (A1 to A3) on one side, DC electrodes (C1 and C2) are formed so as to put both side of each optical waveguide therebetween, and each of DC bias voltages (V1 and V2) is applied to each of the DC electrodes (C1 and C2). Further, at the Mach-Zehnder type optical waveguides (B1 to B3) on the other side, DC electrodes (C3 and C4) are formed so as to put both side of each optical waveguide therebetween, and each of DC bias voltages (V3 and V4) is applied to each of the DC electrodes (C3 and C4).
In the DC electrode of FIG. 1, for example, at locations shown by dotted line E1 and a dotted line E2, the feeder electrode crosses the optical waveguides (A1 and B1). For this reason, some of light waves propagating through the optical waveguide (A1) are absorbed by the feeder electrode, and thus the intensity of light becomes different at the branched waveguide A1 and the branched waveguide A2, and thus this causes degradation of an extinction ratio combining the two lights. The same applies to the optical waveguides (B1 to B3). In particular, in an optical modulator in which a plurality of Mach-Zehnder type optical waveguides are incorporated, as in the nested optical waveguide or the like, an advanced modulation technique such as high-frequency modulation or multi-level modulation is used, and thus even a slight degradation of the extinction ratio causes significantly influencing the characteristics of the optical modulator.
Further, in a case where the power supply to the DC electrode is performed from one side of the optical modulator, or the like, the disposition of the feeder electrode tends to become asymmetric with respect to symmetric axis of the optical waveguide (in the case of the Mach-Zehnder type optical waveguide, a line passing through the middle between two branched waveguides) due to the wiring pattern of the feeder electrode of the DC electrode. In FIG. 1, a feeder electrode is not present in a portion (a dotted line E4) symmetrical to a feeder electrode portion shown by a dotted line E3. Such asymmetry causes different of internal stress of the electrode (the feeder electrode) on the optical waveguide at each optical waveguide, thereby causing further degradation of the extinction ratio due to a change in the mode field diameter of each optical waveguide, or a temperature drift phenomenon (a phenomenon in which the operating point of the Mach-Zehnder type optical waveguide is shifted due to a change in temperature).